Semiconductor memory device

ABSTRACT

A semiconductor memory device which is capable of reducing a delay in the conversion of an input chip enable signal having a TTL level, providing a quick chip enable access and avoiding an increase in current consumption despite the quick chip enable access. The semiconductor memory device in one embodiment includes an input buffer outputting a signal having a CMOS level in response to a chip enable signal having a TTL level, and having a plurality of transistors whose gate lengths are set to first dimensions, and a second input buffer activated in response to both another input signal having a TTL level and the signal having the CMOS level, and having a plurality of transistors whose gate lengths are set to second dimensions greater than the first dimensions.

CROSS REFERENCE TO RELATED APPLICATION

This application claims rights of priority under 35 USC §119 of JapanesePatent application Ser. No. 5-236879, filed Sep. 22, 1993, the entiredisclosure of which is incorporated herein by reference. This is aContinuation of application Ser. No. 08/306,916, filed Sep. 16, 1994,now U.S. Pat. No. 5,500,614.

BACKGROUND OF THE INVENTION

An input buffer included in a conventional SRAM will be described withreference to FIG. 1.

The input buffer serves as a circuit for converting a signal having aTTL (Transistor Transistor Logic) level to a signal having a CMOS(Complementary MOS) level. The input buffer comprises CMOS transistors.Described specifically, the input buffer comprises a PMOS transistor(hereinafter also "PMOS") 1 whose source is electrically connected to apower source potential VCC, an NMOS transistor (hereinafter also "NMOS")2; whose source is electrically connected to a ground potential VSS andan inverter 3. The signal having the TTL level is input to the gates ofthe PMOS 1 and the NMOS 2.

FIG. 2(a) shows the voltages of a signal having a TTL level, which isrepresented as an input signal. FIG. 2(b) shows the voltages of a signalhaving a CMOS level, which is represented as an output signal.

The operation of the input buffer shown in FIG. 1 will now be describedwith reference to FIGS. 2(a) and 2(b).

When a signal having a TTL level of 0.8 V is first input to the inputbuffer as an input signal having an "L" level, the PMOS 1 is turned onand the NMOS 2 is turned off. As a result, the level of a signal outputfrom the input buffer through the inverter 3 is brought to the groundpotential VSS (0 V), for example.

When a signal having a TTL level of 2.2 V is input to the input bufferas an input signal having an "H" level, the NMOS 2 is brought into aconducting state. Thus, the level of a signal output from the inputbuffer through the inverter 3 is brought to a power source potential VCC(5 V), for example. As a result, a signal having the TTL level isconverted to a signal having the CMOS level as shown in FIGS. 2(a) and2(b).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductormemory which is device capable of reducing a delay in the conversion ofan input chip enable signal having a TTL level, providing a quick chipenable access, and avoiding an increase in current consumption eventhough the quick chip enable access is made possible.

In order to achieve the above object, a semiconductor memory deviceaccording to the invention comprises an input buffer outputting a signalhaving a CMOS level in response to a chip enable signal having a TTLlevel, wherein the input buffer has a plurality of transistors whosegate lengths are set to first dimensions. The memory device alsocomprises a second input buffer activated in response to both anotherinput signal having a TTL, level and the signal having the CMOS level,wherein the second buffer has a plurality of transistors whose gatelengths are set to second dimensions greater than the first dimensions.Further, a semiconductor memory device according to the inventioncomprises an input buffer outputting a signal having a CMOS level inresponse to a chip enable signal having a TTL level, wherein the inputbuffer has a plurality of transistors whose gate widths are set to firstdimensions. The memory device also comprises a second input bufferactivated in response to both another input signal having a TTL level,and the signal having the CMOS level, wherein the second input bufferhas a plurality of transistors whose gate widths are set to seconddimensions smaller than the first dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a conventional input buffer;

FIG. 2A and FIG. 2B showing a view for describing the operation of theconventional input buffer shown in FIG. 2;

FIG. 3 is a block diagram showing the structure of a semiconductormemory device according to the present invention; and

FIG. 4 is a view for describing the operation of the semiconductormemory device shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention has been achieved based on the ideas andfoundations to be described later.

A input buffer for converting a signal having a TTL level to a signalhaving a CMOS level can provide a high-speed converting operation owingto an increase in the dimensions of the gate widths of the transistorswhich form the input buffer. However, the current consumption of theinput buffer increases with an increase in the dimensions of the gatewidths. Therefore, the dimensions of the gate widths of the transistorsforming a plurality of input buffers included in a semiconductor memorydevice have been varied in consideration of performance priorities.Thus, the construction of a semiconductor memory device which is capableof providing a high-speed converting operation and capable of decreasingthe current consumption has been derived from the result of such aprocess.

Factors for determining performance will be described below in detail.

Let us now consider the factors for determining the operating speed.There are some cases where the speed for converting a signal having aTTL level to a signal having a CMOS level varies depending on how thepotential level of the input signal varies. When a change from "L" to"H" of the potential level of the input signal having the TTL level iscompared with a change of the potential level thereof from "H" to "L",the latter change requires an increased period of time to convert thesignal having the TTL level to the signal having the CMOS level. This isbecause an input potential corresponding to "H" of the signal having theTTL level is lower than an input potential corresponding to "H" of thesignal having the CMOS level, owing to the fact that the input buffercomprised of a CMOS circuit unbalances the performance of PMOS and NMOStransistors to make it easy to detect "H" of the signal having the TTLlevel. Such a situation will be described with reference to FIG. 1, forexample. If a CMOS level is input as an input voltage when thesemiconductor memory device is activated by a power source voltage of 5V, then the level of the input voltage is set to "H" or "L" with 2.5 Vcorresponding to one-half the power source voltage as the center.Therefore, PMOS 1 and NMOS 2 are substantially identical in performanceto each other and produce outputs at the same speed regardless ofwhether the level of the input voltage is "H" or "L". However, if a TTLlevel is input as an input voltage, then the level of the input voltageis set to "H" and "L" with 1.5 V as the center. Therefore, the PMOStransistor 1 is reduced in performance as compared with the NMOStransistor 2. Accordingly, the speed required to output "H" becomes slowand the speed required to output "L" becomes fast. Namely, when theinput having the TTL level changes from "L" to "H", the output speedbecomes faster.

Let's next consider factors for determining the current consumption.When a potential level of an input signal having a TTL level is "H" or"L", a stationary current which flows in the input buffer, often varies.When the potential level of the input signal having the TTL level is"H", the stationary current which flows in the input buffer increases ascompared with the case where the potential level of the input signal is"L". This is because the current continues flowing due to the fact thatwhen the potential level of the input signal having the TTL level is"H", its potential level is lower than the power source voltage appliedto the CMOS circuit and the PMOS forming the input buffer is not fullyturned off. This situation will be described with reference to FIG. 1,for example. When a potential level "H" (2.2 V) is input to an input T₁as the input signal having the TTL level, the NMOS 2 is turned onbecause the source and gate thereof have been supplied with a groundlevel voltage GND (0 V) and 2.2 V respectively. Since the source andgate of the PMOS 1 are respectively supplied with a power source voltageVCC (5 V) and 2.2 V, the PMOS 1 is not turned off. Therefore, thecurrent flows in the PMOS 1.

As other factors for determining performance include the order andtiming used for each input signal.

It is apparent from the above description that increasing the outputspeed and controlling an increase in current consumption, which takesplace with an increase in the output speed, can be achieved by settingthe gate widths of transistors forming an input buffer which is suppliedwith a chip enable signal of a negative logic so as to be wider than thegate width dimensions of transistors forming an input buffer which issupplied with other input signals (such as a chip enable signal of apositive logic, an address signal, a data input, a write enable signalor an output enable signal), or setting the gate length dimensions so asto be shorter than those of the gate length of the transistors formingthe input buffer supplied with the input signals described above.

The semiconductor memory device will now be described specifically bytaking a SRAM as an example.

In the semiconductor memory device, particularly a low currentconsumption type SRAM, a chip select operation causes a delay in theoperation of the SRAM itself. This is because a chip enable signalbrings an inner circuit, including input buffers each supplied with anaddress signal and a memory cell array, into a non-selecting state toreduce the current consumption at the time that a chip is in anon-selected state and thereafter the SRAM starts to operate for chipselection after the chip enable signal has been input. The operation fortaking the chip enable signal in the SRAM is made faster to increase theoperation of the SRAM itself. Further, when a negative-logic chip enablesignal performs a changeover of the chip from a non-selected state to aselected state, a change of the potential level of the input from "H" to"L" is slow. Therefore, the SRAM needs to operate at a higher speed inorder to make the change in the level of the input faster. Thus, thepriority given for increasing the dimensions of the gate widths of thetransistors forming the input buffer is highest. Since the chip enablesignal is brought to "L" upon selecting the chip, the current whichflows upon the operation of the SRAM, hardly increases even if thedimensions of the gate widths are made greater. Therefore, the currentconsumption can be lowered. Even in the case of non-selection of thechip, there is no current increase so long as a signal having the CMOSlevel is input. When positive-logic and negative-logic chip enablesignals are input, the positive-logic chip enable signal is used so asto deactivate the input buffer supplied with the negative-logic chipenable signal. When this is done, an increase in the current consumptiondoes not occur even in a case where the signal having the TTL level isinput.

Let's now consider the positive-logic chip enable signal. In this case,the positive-logic chip enable signal becomes faster in operation thanthe negative-logic chip enable signal, upon changing the potential levelof the input from "L" to "H" to switch the chip from the non-selectedstate to the selected state. Therefore, the priority given forincreasing the dimensions of the gate widths of the transistors formingthe input buffer is a second priority. Since the potential level of theinput positive-logic chip enable signal is brought to "H" upon selectingthe chip, the current which flows when the SRAM is operated increases ifthe dimensions of the gate widths are enlarged. It is therefore notpossible to increase their dimensions.

Let's next consider the address signal. Since the address signal isinput to the input buffer after the chip has been selected by a chipenable signal, the priority level given for increasing the dimensions ofthe gate widths of the transistors forming the input buffer is a thirdpriority. The more the dimensions increase, the more the current thatflows when the SRAM is operated increases.

Let's further consider the data input, the write enable signal and theoutput enable signal. Since the data input, the write enable signal andthe output enable signal are needed after the address signal has beendetermined, the priority level given for increasing the dimensions ofthe gate widths of the transistors forming the input buffer is loweredas compared with the priority levels determined upon using thenegative-logic and positive-logic chip enable signals and the addresssignal. It is therefore necessary to decrease their dimensions andreduce the current flowing upon the operation of the SRAM.

A semiconductor memory device according to the present invention willnow be described in more detail with reference to FIG. 3.

The semiconductor memory device is a SRAM comprised of CMOSs, forexample, which are selectively activated based on a plurality of chipenable signals.

The semiconductor memory device shown in FIG. 3 comprises an inputbuffer 10 for selectively converting a negative-logic chip enable signalCE1 having a TTL level to a signal having a CMOS level, an input buffer20 for receiving therein a positive-logic chip enable signal CE2corresponding to a signal complementary to the signal CE1 when thesemiconductor memory device is in operation, an input buffer 30activated so as to transfer input data D_(in), an input buffer 40 forreceiving a negative-logic write enable signal WE therein, an inputbuffer 50 for receiving a negative-logic output enable signal OE thereinand an input buffer unit 60 having a plurality of input buffers forreceiving addresses ADR for a memory cell array therein. Further, thesemiconductor memory device according to the present invention is alsoprovided with a NAND circuit 72. The NAND circuit 72 activates each ofthe input buffers 30, 40, 50 and 60 in response to the output of theinput buffer 10, and the output of the input buffer 20, which issupplied to the NAND circuit 72 via an inverter 71.

Further, the semiconductor memory device according to the presentinvention includes an inner circuit 100 activated in response to signalsinput thereto from the input buffers 30, 40, 50 and 60. The innercircuit 100 comprises a memory cell array 110 for the SRAM, a rowaddress decoder 120, a column address decoder 130, an input data controlcircuit 140, a R/W input/output circuit 150 and a data output buffer160. The memory cell array 110 for the SRAM is comprised of CMOSs orNMOSs and retains data therein. The row address decoder 120 decodes arow address. The column address decoder 130 decodes a column address.The input data control circuit 140 performs switching of the data D_(in)supplied from the input buffer 30. The R/W input/output circuit 150inputs write data or read data to a memory cell selected by the decoders120 and 130 and outputs the write or read data from the memory cell. Thedata output buffer 160 is activated in response to the output enablesignal OE, the chip enable signal CE and the write enable signal WE andoutputs read data to the outside based on these signals.

The input buffer 10 is comprised of a two input NAND gate and issupplied with the chip enable signal CE1 having the TTL level from aninput terminal T10. The input buffer 10 converts the signal CE1 havingthe TTL level to a signal having a CMOS level.

The input buffer 10 comprises a PMOS 11 and NMOS 12 and a PMOS 13 and anNMOS 14 which form CMOSs. The gates of the PMOS 11 and the NMOS 12 areelectrically connected to each other and to input terminal T10. Further,the drains of the PMOS 11 and the NMOS 12 are electrically connected toeach other. The source of the PMOS 11 is electrically connected to apower source VCC. The source of the PMOS 13 is electrically connected tothe power source VCC in parallel with the source of the PMOS 11. Thesource of the NMOS 14 is electrically connected to a ground potentialVSS. The drains of the PMOS 13 and the NMOS 14 are electricallyconnected to each other through the NMOS 12.

The input buffer 20 serves as a circuit for receiving therein the signalCE2 having the TTL level, which has been inverted with respect to thechip enable signal CE1 upon selecting the chip and converting the levelof the signal CE2 to a CMOS level. The input buffer 20 is made up of aCMOS inverter comprised of a PMOS 21 and an NMOS 22.

The input buffer 30 comprises a NAND gate having two inputs, one ofwhich is supplied with the data D_(in) having the TTL level through aninput terminal T30 and the other of which is supplied with the signal CEoutput from the NAND circuit 72. The input terminal T30 is electricallyconnected to the gate electrodes of a PMOS 31 and an NMOS 32 whosedrains are electrically connected to each other. The source of the NMOS32 is electrically connected to the ground potential VSS. The output ofthe NAND circuit 72 is input to the gate electrodes of the PMOS 33 andNMOS 34 in common. The source of the NMOS 34 is electrically connectedto the ground potential VSS in parallel with the source of the NMOS 32.The source of the PMOS 33 is electrically connected to the power sourceVCC. The drains of the PMOS 33 and the NMOS 34 are electricallyconnected to each other through the PMOS 31. Each of the input buffers40, 50 and 60 for respectively converting the write enable signal WE,the output enable signal OE and the addresses ADR each having the TTLlevel to the CMOS levels, is identical in structure to the input buffer30. Further, the input buffers 40, 50 and 60 respectively havetransistors 41 through 44, transistors 51 through 54 and transistors 61through 64.

The dimensions of the respective transistors 11 through 14 in the inputbuffer 10 are set so as to be larger than the dimensions of thetransistors 21 and 22 in the input buffer 20 or the dimensions of thetransistors 81 through 34, 41 through 44, 51 through 54 and 61 through64 in the input buffers 30 through 60. Namely, the gate widths of therespective NMOSs or PMOSs in the input buffer 10 are set larger thanthose of the respective NMOSs and PMOSs in each of the input buffers 20through 60 or the gate lengths thereof are set shorter than thosethereof to increase the current that flows in the transistors.

The operation of the semiconductor memory device according to thepresent invention, which is shown in FIG. 3, will now be described belowwith reference to FIG. 4.

Upon selecting the chip, a TTL level "L" is input to a CE1 terminal anda TTL level "H" is input to a CE2 terminal. Further, the TTL level "H"or "L" is input to each of the ADR, D_(in), WE and OE terminals. Whenthe TTL level "L" is first input to the CE1 terminal, an "H" convertedto CMOS level is produced as the output of the input buffer 10.Similarly, when the TTL level "H" is input to the CE2 terminal, an "L"converted to CMOS level is produced as the output of the input buffer 20and is brought to an "H" by the inverter 71.

Since the potential level input to the input buffer 20 changes from the"L" to the "H", the speed of converting the input potential level fromthe TTL level to the CMOS level by the input buffer 20 is faster thanthat by the input buffer 10. The NAND circuit 72 outputs an "L" as theoutput CE in synchronism with either one of the outputs produced fromthe input buffer 10 and the inverter 71, which is delayed in phasebetween the two. When a chip select signal is simultaneously input tothe CE1 and CE2 terminals, the NAND circuit 72 outputs an "L" insynchronism with the output of the input buffer 10. Thus, the output "L"is supplied to the NAND circuit 72 at rapid speed in the case of thepresent invention in which the speed at which the output of the inputbuffer 10 is produced has been increased. A time interval t1 in FIG. 4becomes faster or shorter. The output signal "L" of the NAND 72 is inputto each of the PMOSs 33, 43, 53 and 63 of the input buffers 30, 40, 50and 60, so that the respective input buffers are activated. The outputsof the activated input buffers 30, 40, 50 and 60 are sent to theircorresponding circuits in the inner circuit 100. As a result, thesemiconductor memory device is operated so as to write data into acorresponding memory cell and read it therefrom.

During the read operation, the addresses ADR are input to the rowaddress decoder 120 and the column address decoder 130. A one-bit memorycell is selected from the memory cell array 110 based on the results ofdecoding the input addresses ADR. Data stored in the selected memorycell is supplied to the R/W input/output circuit 150. Next, the data isoutput to the outside through the data output buffer 160. Here, a chipenable access time corresponds to a time interval t₂ in the drawing.

During the write operation, the addresses ADR are input to the rowaddress decoder 120 and the column address decoder 130. Based on theresults of decoding the input addresses ADR, a one-bit memory cell isselected from the memory cell array 110. The data D_(in) is input to theinput data control circuit 140 where it is controlled based on the writeenable signal WE. The so-processed data D_(in) is next sent to the R/Winput/output circuit 150 where it is written into its correspondingmemory cell selected from the memory cell array 110.

When the chip is in a non-selected state, an "H" is input to the chipenable signal CE1 terminal. Alternatively, an "L" is input to the chipenable signal CE2 terminal. Therefore, each of the input buffers 30, 40,50 and 60 and the inner circuit 100 is brought into a deactivatingstate. When the "L" is input to the chip enable signal CE2 terminal, theinput buffer 10 is also brought into the deactivating state.

While the present invention has been described with reference to theillustrative embodiment, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiment will be apparent to those skilled in the art on reference tothis description. It is therefore contemplated that the appended claimswill cover any such modifications or embodiments as fall within the truescope of the invention.

What is claimed is:
 1. A semiconductor memory device comprising:a firstinput buffer that outputs a CMOS level signal in response to a first TTLlevel input signal, said first input buffer having a first transistor ofa first conductivity, the first transistor having a gate to which thefirst TTL level input signal is input; and a second input buffer thatresponds to a second TTL level input signal and the CMOS level signal,said second input buffer having a second transistor of the firstconductivity, the second transistor having a gate to which the secondTTL level input signal is input; wherein a gate length of the secondtransistor is greater than a gate length of the first transistor.
 2. Thesemiconductor memory device as claimed in claim 1, wherein the first TTLlevel input signal is a negative logic signal.
 3. The semiconductormemory device as claimed in claim 1, further including an inner circuitthat is responsive to the CMOS level signal and having a memory cellarray.
 4. The semiconductor memory device as claimed in claim 1, furthercomprising a third input buffer that is responsive to the first andsecond TTL level input signals.
 5. The semiconductor memory device asclaimed in claim 1, wherein the memory device is an SRAM.
 6. Thesemiconductor memory device as claimed in claim 3, wherein said innercircuit includes a memory cell array, a row address decoder, a columnaddress decoder, an input data control circuit, a R/W input/outputcircuit, and a data output buffer.
 7. The semiconductor memory device asclaimed in claim 1, wherein the first and second transistors areselected from the group consisting of NMOS transistors and PMOStransistors.